In LTE and LTE-A systems, downlink (DL) control signals transmitted from a network base station (i.e. from an evolved NodeB or “eNodeB”) may be User Equipment (UE)-specific (that is they may be specific to a particular “user equipment” such as a particular wireless terminal or portable communication device), or they may be cell-specific (i.e. broadcast). A UE-specific control signal provides scheduling assignment (SA) for DL data reception at the specific UE and/or uplink (UL) grant for data transmission from the specific UE. Broadcast or cell-specific signalling conveys system information to a group of UEs or all UEs within the cell.
Generally, DL control information (DCI) can be conveyed by Physical Downlink Control Channel (PDCCH) and/or enhanced PDCCH (EPDCCH). It is possible for a transmission to be made using the PDCCH or EPDCCH with associated Physical Downlink Shared Channel(s) (PDSCH(s)). For example, in UE-specific control signalling, the PDCCH carries downlink control information (DCI) for the reception, demodulation and decoding of the associated PDSCH/DL-SCH (DL-SCH stands for Downlink Shared Channel).
It is also possible for transmissions to be made using the PDCCH or EPDCCH alone without associated PDSCH(s). Such so-called “standalone” PDCCH or EPDCCH is used, for instance, in LTE Release 10 (Rel.10) for semi-persistent scheduling (SPS) activation and release. DCI format 0/4, on the other hand, although also without associated PDSCH/DL-SCH transmission, is used for providing UL grant to a UE on UL-SCH/PUSCH (Uplink Shared Channel/Physical Uplink Shared Channel) transmission in the later subframe.
The PDCCH or EPDCCH is capable of broadcasting different types of system information such as System Information Block Type n (SIB-n, n=1, . . . , 13) and/or paging etc. In order to inform a UE of the usage of one PDCCH received in the DL, a specific 16-bit sequence is used to scramble the 16 Cyclic Redundancy Check (CRC) bits of a DCI payload at the eNodeB before transmission, and the UE tries different scrambling sequences after a successful blind decoding. The UE knows the usage of the DCI (e.g. for paging) when the CRC check passes after descrambling using one scrambling sequence (e.g. Paging Radio Network Temporary Identifier (P-RNTI)).
In real systems, false positive detections may occur when a CRC check passes even though it should not. In other words, there is a possibility of a UE falsely/incorrectly considering a PDCCH to be destined to itself. This may occur where the CRC check of the PDCCH by a UE is correct (positive) even though the PDCCH was not, in fact, intended for that UE, i.e. the CRC passes even though there is a UE identifier mismatch. These so-called “false positives” may occur if the effects of transmission errors caused by the radio channel and UE identifier mismatch cancel each other. In other words, such “false positives” can be caused by decoding errors whereby error bits in the DCI bit string and CRC are aligned and allow the CRC check to pass.
Compared to “standalone” PDCCH or EPDCCH (described above), systems in which transmissions are made using the PDCCH or EPDCCH with associated PDSCH(s) may be less impacted by false positive detection. For instance, in systems in which transmissions are made using the PDCCH or EPDCCH with associated PDSCH(s), in the event of a false positive, the UE will mistakenly interpret the information in the detected DCI and try to decode the associated PDSCH(s). It may be possible for the UE to receive but it will unsuccessfully decode the associated PDSCH(s)/DL-SCH. As a result, the UE will either feedback a negative-acknowledge (NAK) if it is a unicast, or discard the received information and try to receive the information in the next interval if it is system broadcast. On the other hand, for “standalone” PDCCH or EPDCCH (i.e. without associated PDSCH(s)/DL-SCH transmission), in the event of a false positive detection, the UE will directly follow the instruction in the detected DCI leading to an incorrect system procedure.
Furthermore, during the process of Rel.8 standardization, “standalone” PDCCH is used for SPS activation/release. This can suffer from false positive detection (as discussed above) and the consequences depend on whether the scheduling assignment (SA) is interpreted as a DL SA or as an UL SA. If a UE incorrectly determines that it has a DL SA, it will fail to decode the presumed data packet transmission from the eNodeB and it will periodically transmit a NAK in the UL of the communication system. This NAK may collide with a NAK or (positive) acknowledge (ACK) transmitted by another UE which has correct PDSCH reception. This is problematic when the UE with the valid PDSCH reception transmits an ACK. Similarly, when an SPS UE incorrectly determines that it has a UL SA; it will transmit data in the UL, which will interfere with data transmitted by one or more other UEs with valid SAs.
In order to reduce the probability of false positive SPS activation for SPS UEs, it has previously been proposed to virtually increase the length of the CRC by setting special fields in the DCI to predetermined values. For example, in US 2010/0070815 A1 it is proposed to set an information element (IE) in a scheduling assignment with determined value and thus extend the effective length of the CRC.